Permafrost oil-production method

ABSTRACT

ANNULUS AND THE PRODUCTION STRING. PREFERABLY THERE IS AN INSULATION LAYER BETWEEN THE INNER ANNULUS AND THE PRODUCTION STRING.   HOT OIL IS PRODUCED FROM A WELL PASSING THROUGH PERMAFROST BY CIRCULATING A COLD FLUID DOWN AN OUTER ANNULUS IN CONTACT WITH THE PERMAFROST AND RETURNING THE FLUID TO THE SURFACE VIA AN INNER ANNULUS IN CONTACT WITH THE OUTER

March 20,. 1973 W. G. CORBETT ETAL PERMAFROST OIL-PRODUCTION METHOD Filed Oct. 6. 1970 \NVENTORSI WILLIAM GEORGE CORBETT MICHAEL WARD c u: a av.- ,F ,wLMam 'THE'|R ATTORNBY5 United States Patent 3,721,298 PERMAFROST OIL-PRODUCTION METHOD William George Corbett, 104 Handside Lane, Welwyn Garden City, Hertfordshire, England, and Michael Ward Clegg, 3 Saville Gardens, Prior Road, Camberley, Surrey, England Filed Oct. 6, 1970, Ser. No. 78,395 Claims priority, application Great Britain, Nov. 10, 1969, 54,891/ 69 Int. Cl. E21b 43/00 US. Cl. 166-302 1 Claim ABSTRACT OF THE DISCLOSURE Hot oil is produced from a well passing through permafrost by circulating a cold fluid dOWn an outer annulus in contact with the permafrost and returning the fluid to the surface via an inner annulus in contact with the outer annulus and the production string. Preferably there is an insulation layer between the inner annulus and the production string.

This invention relates to an oil well.

Oil has recently been found under the north slope of Alaska and in this location (and also in other locations) the earths surface consists of a material usually known as permafrost. In this specification the term permafrost will be used to denote a geological formation which contains ice in such quantities that the mechanical properties of the formation depend upon the ice so that there would be a substantial change in the mechanical properties should the formation heat to a temperature at which the ice melts and this definition extends to a formation which consists of ice. In Alaska the permafrost consists of a mixture of gravel and ice.

Oil usually issues from a production well at a temperature above 32 F. and there is a danger of melting the ice in the permafrost which surrounds a production well. Since converting ice to water reduces its volume melting the ice in the permafrost will cause subsidence and this subsidence could impose a compressive load high enough to fracture the well casing and hence allow the escape of oil.

It is an object of this invention to reduce the contamination risk of an oil production well passing through permafrost.

According to the invention an oil production well which passes through permafrost comprises an outer casing in contact with the permafrost and a second casing situated inside said outer casing and separated therefrom so as to form an annular space, the well also comprising a separator casing situated in the annular space and dividing it into an inner and an outer zone so as to provide a return flow path through which a cold liquid can be circulated so as to reduce heat transfer to the permafrost.

In a preferred embodiment the well comprises at least one inner casing situated inside the second casing so as to create at least one annulus adapted to contain thermal insulation.

The invention also comprises a method of producing oil at a temperature above freezing point from a well which passes through permafrost, in which a cold liquid is passed from the surface to below the level of the permafrost via an outer annular zone whose outer wall is an outer casing in contact with the permafrost and returned to the surface through an inner annular zone whose outer wall forms the inner wall of the outer annular zone, the temperature at which the cold liquid enters the outer annular zone being low enough to ensure that its temperature remains below freezing point until it has passed below the level of the permafrost.

The invention will now be described by way of example with reference to the drawing accompanying the provisional specification which shows diagrammatically a cross section through an Alaskan production well.

The well illustrated in the drawing comprises an outer casing 10 (of diameter 18% in.) which extends from the surface to 2300 ft., i.e. to just below the base 25 of the permafrost 13. This casing is cemented to the permafrost with a cement layer 14.

A second casing 11, diameter 13% in., extends from the surface to 4200 ft. Its upper section is situated inside the outer casing 10 so as to create an annulus 15. Below the permafrost the second casing 12 is cemented to the formation.

Similarly an inner casing 12, diameter 9% in., extends from the surface to 9100 ft., i.e. to the level of the producing formation. The uppermost 4200 ft. of the cas1ng 12 is situated inside the second casing 11. The inner casing 12 is cemented to the second casing 11 below the level of the permafrost and to the formation below the level of the second casing 11. (The well is not illustrated below the permafrost since it conforms with normal pract1ce at these levels.)

The annulus 15, which is situated at permafrost level, 18 divided into an outer zone 14 and an inner zone 17 by means of aseparator casing 18 so as to provide an outer down path and an inner up path for cold llqllld.

The well also comprises a second inner casing 19 which is sealed to the inner casing 12 by means of a conventional packer 20. Thus there are two annular spaces, 21 and 22, and these are filled with thermal 1nsulat1on, e.g. expanded polyurethane.

As it issues from the production well the 011 is at a temperature of about F. and to reduce heat conduction to the permafrost a pump 23 circulates a cold liquid, e.g. an oil of low enough pour point, down the outer zone 16, up the inner zone 17, to a refrigerator 24 to remove the heat gained during circulation and then back to the pumps 17. The initial temperature is low enough to ensure that the cold liquid remains below freezing point at the bottom of the annulus 15 and it is clear that this avoids the danger of melting the permafrost. The conduction of heat from the hot oil inside the second inner casing 19 to the cold liquid in the inner zone 17 is reduced by the thermal insulation in the annular spaces 21 and 22. This reduces the power consumption needed to cool the cold liquid.

In addition to the features illustrated in the drawing the well comprises a conventional tubing string and well head equipment in accordance with normal practice. All the casings (10, 11, 12, 18 and 19) are suspended from the well head equipment so that the upper portions remain in tension.

We claim:

1. A method of producing oil at a temperature above the freezing point of water from a well which passes through Pfifmafrost, omprising passing a cold liquid from the surface to below the level of the permafrost via an outer annular zone whose outer wall is an outer casing positioned in said well in contact with the permafrost and 1,812,267 6/1931 Lewis 166-302 UX returning said liquid to the sunface through an inner an- 3,142,336 7/1964 Doscher 166-57 X nular zone whose outer wall is a casing in the well which 3,217,791 11/ 1965 Long 1 66-DIG. 1 also forms the inner wall of the outer annular zone, While 3,380,530 4/1968 McConnell et a1. 166303 producing said oil through a casing enclosed by said inner 5 3,456,735 7/ 1969 McDougall et a1. 16657 UX zone, the temperature at which the cold liquid enters 3,498,381 3/ 1970 Earlougher 166303 the outer annular zone being low enough to ensure that 3,613,792 10/1971 Hyde 166-315 its temperature remains below the freezing point of water FOREIGN PATENTS until 1t has passed below the level of the permafrost. 1,842,205 V1967 USS-R. "nu" 166 DIG 10 References Cited UNITED STATES PATENTS 3,561,531 2/1971 Miller 166DIG.1 1,342,780 6/1920 Vedder 166302 X 15 ROBERT L. WOLFE, Primary Examiner mg?- UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,721,298 Deted March 20 1223 Inventofls) W.G. Corbett et a1 I v i It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

for "14" read 16 Co1. 4,.Line 10,

' I for 1,842,205 I read 184,205

In Column 1, line 6, after "England", Assignor to The British Petroleum'company Limited, London, England should be added.

Signed and sealed this 10th day of December 1974.

(SEAL) Attest: McCOY M. GIBSON JR. 0. MARSHALL'DANN Attesting Officer Commissioner of Patents 

